In recent years, there has been a trend toward replacing large, mainframe computer systems with networks of personal computers (PCs). Despite this downsizing trend, U.S. businesses have billions of dollars invested in applications running on mainframe computer systems. Replacing these mainframe computer systems is not always cost effective. Further, many engineering and scientific applications still require the computing power of large mainframe systems. Thus, mainframe computer systems are likely to continue to make up a significant segment of the computing industry for years to come.
In a typical mainframe system, people interact with the mainframe computer through terminals. The terminals include screens and keyboards for interacting with the mainframe. Applications running on the mainframe computer may serve multiple terminals simultaneously. Similarly, a single terminal can have multiple sessions running on the mainframe at the same time.
IBM's System Network Architecture (SNA) is a scheme for connecting terminals, printers, and other devices with a mainframe computer. In a typical SNA system, each terminal is connected to a terminal cluster controller through a coaxial cable. The cluster controller acts as a concentrator by gathering messages from multiple terminals for more efficient transmission to the mainframe. Groups of cluster controllers connect via a telecommunications line to a communications controller or front-end processor (FEP). The front-end processor concentrates signals before they are transmitted to the mainframe.
In the SNA connection scheme, each terminal or printer is called a physical unit (PU). Each PU holds one or more logical units (LUs). The logical units address and interact with the mainframe in an SNA system. IBM's Virtual Telecommunications Access Method (VTAM) software, which runs in the mainframe, cooperates with the front-end processor to communicate with the LUs.
Today, large numbers of people have personal computers on their desk top. These desk-top computers are now capable of handling many tasks which were once possible only with mainframe computers and minicomputers. However, many people with desk-top computers also need access to data or programs running on a mainframe computer. It is neither practical nor economical to have both a desk-top computer and a mainframe terminal on each person's desk top.
Terminal emulation products are available which enable a PC or desk-top computer to emulate a mainframe terminal. With these terminal emulation products, PCs can be connected to and share data with mainframe computers. One common method of connecting PCs with mainframe computers is by direct connection of the mainframe to a token ring local area network (TR LAN). In recent years, IBM gave its cluster controllers and front-end processors the capability to become nodes on a token-ring network. Since PCs can also connect directly to a token-ring network, PCs and mainframes can interact on the same network.
A token ring network uses a physical hub topology. In this arrangement, the cables or wires run from a central hub to each station. Each hub connection is referred to as a port. A simple network with less than eight stations may have only a single hub. Larger networks usually have multiple hubs which are connected to one another. One significant advantage of the hub topology is that it isolates the wires connecting each station on the network so that if a failure occurs, the rest of the network can continue to operate. This topology also makes it easy to add, move, and remove PCs and other devices to the network.
The ability to easily reconfigure the physical topology of the network can also be troublesome. When problems occur in a large network, technicians need to correlate the logical address of devices on the network with their physical location in order to isolate the problem. This information is usually in the form of manually created charts, tables or diagrams which must be continuously updated by a network administrator. Since some networks can have as many as 250,000 devices, maintaining such records can be a mammoth undertaking involving hundreds of network administrators.